Automated train control systems require accurate, reliable train location information to safely facilitate train regulation (spacing, merging, diverting, etc.). The basis for much of today's automated train control systems is an automated block system, in which the railway track is subdivided into predetermined segments (blocks), and where the presence of a train within a block is detected and appropriate detection control signals formed. The detection control signals are then provided to a signalling/control system which, at a minimum, provides visual indicators to other trains that the block is currently occupied. In more elaborate signalling/control systems, trains can be automatically prevented from entering an occupied block.
The safety of the automated train control system is dependant upon reliable detection of trains within a block. Failure of such detection can result in dangerous, and potentially deadly, accidents. In a first known method (herein referred to as "shunting method") for detecting the presence of a train within a block, each block of track is electrically insulated from the remainder of the track. A relay is connected across the tracks at the end of the block and a voltage is supplied across the tracks at the beginning of the block. Accordingly, while no train is present within the block, the relay is energized thereby forming a block empty detection control signal. However, when a train enters the block, the tracks are short circuited by the wheels and axles of the train, and therefore, the relay is de-energized to form a block occupied detection control signal. This shunting method of detection is described in greater detail in "Safety and Automatic Train Control for Rail Rapid Transit Systems", Jul. 1974, Report No. DOT-TSC-OST-74-4, pgs. B11-B13.
The shunting method is disadvantaged in that loss of shunt, even when the train is within a block, can be experienced for a number of reasons. The loss of shunt may be caused by the formation of an oxide film on the wheel or rail or by debris on the rails, such as fallen leaves, grease, resin, or brake shoe shavings. While these conditions for failure of the shunt method have always existed, advances in train technology have increased the likelihood of such failures. The tracking of newer trains has progressed to such a point that sideways motion of the trains, which traditionally allowed the side of the train wheels to scrape against the tracks thereby removing insulating films or oxide layers, is decreased and the insulating films and oxide layers build up. Further, lighter weight train cars may not exert enough pressure to enhance wheel rail electrical contract.
A second known method of detecting trains within a block (hereinafter "inductive impedance method") utilizes insulated wire loops installed between the rails. An impedance bridge is used to measure the impedance of the loop; the value being dependent on whether or not a large metal object, such as a train occupies the track above the loop. The size of each wire loop, due to various electrical limitations, are generally limited in size to approximately 100 feet, and since a block may be much larger than the wire loops (e.g., depending upon the anticipated train speeds a blocks can be miles long) multiple loops, and associated logic circuits, are required to cover each block. This method is known to be susceptible to external noise sources. This inductive loop method is described in greater detail in "Safety and Automatic Train Control for Rail Rapid Transit System", Jul. 1974, Report No. DOT-TSC-OST-74-4, pg B13.
A still further method of detecting trains is herein referred to as signal blocking. In this method, a distributed transmitting antenna is positioned on one side of the track over the full length of the block and a distributed receiving antenna is positioned on the other side of the track over the full length of the block. The transmitting antenna radiates at frequencies ranging from 1Mhz and up. As a train enters the block, the received signal is attenuated and appropriate detection control signals formed. Embodiments are also known where discrete transmitting antennas and receiving antennas are paired and are distributed over the length of the block. Appropriate logic circuits are provided to generate detection control signals when several receivers detect attenuation of the radiated signals. The signal blocking method is sensitive to environmental conditions, and further, requires that protection be provided against vandalism.
A still further known method of detecting a train within a block is herein referred to as "reflection signalling". This system is similar to the signal blocking method but differs in that the transmitter and receiver are on the same side of the track. As a train passes, the receiver detects energy reflected from the train and forms an appropriate detection control signal. This reflection signalling method is similar to the signal blocking method in that it is sensitive to environmental conditions and also requires that protection be provided against vandalism. Both the signal blocking method and the reflection signalling method are described in greater detail in "Safety and Automatic Train Control for Rail Rapid Transit Systems", July 1974, Report No. DOT-TSC-OST-74-4, pgs B13-B16.
Wheel sensor can also be used for detecting the presence of a train at a predetermined location within a block. For example, U.S. Pat. No. 3,721,821 describes a railway wheel sensor for detecting passage of a railway wheel. However, wheel detection systems, and other such detection system which monitor a specific point or a relatively small area, are inadequate for detecting the presence of a train within a large block unless multiple detectors are placed along the length of the block and appropriate logic circuits employed, thereby greatly increasing the complexity of the block system.
Highly sensitive fiber optic magnetometers for detecting very small magnetic fields are known. U.S. Pat. Nos. 4,603,296, 4,442,350, 4,378,497, 4,918,371, and 4,609,871, which are hereby incorporated by reference, describe various such magnetometers. Fiber optic magnetometers generally have a sensor arm which includes a first fiber optic cable having a magnetostrictive jacket, and a second fiber optic cable, without a magnetostrictive jacket, as a reference arm. A light source is projected through both the reference arm and the sensor arm. Magnetic fields imposed on the sensor arm cause the magnetostrictive jacket to constrict and thereby change the length and dimensions of the fiber optic cable in the sensor arm. Accordingly, when magnetic fields are present, due to the physical changes to the fiber optic cable in the sensor arm, the phase of the light in the sensor arm shifts relative to the phase of the light in the reference arm (which does not vary as a function of the applied magnetic field), and such variations in phase correspond to the magnitude of the magnetic field. Various known circuits are describe for detecting and measuring the resulting phase shifts.
Prior known high sensitivity fiber optic magnetometers have relatively short sensor heads (less than one meter) and require auxiliary current-carrying solenoids over the length of the sensor head. The purpose of the current carrying head, as described in an article entitled "Recent Developments in Fiber Optic Magnetostrictive Sensors", Proc. S.P.I.E. -Int. Soc. Opt. Eng. (USA), Vol. 1367, pp. 226-235 (1991), which is hereby incorporated by reference thereto, is threefold. First, it provides a D.C. bias magnetic field to optimize the magnetostrictive output. Second, a high frequency "dither" magnetic field reduces the effects of thermal noise. Third, a D.C. magnetic feedback field provides great linearity.
It should be noted, however, that the known fiber optic magnetometers are generally point detection magnetometers, in that the sensing arm and the reference arm are contained in the short sensor head, and are not distributed over a long distance. To detect magnetic fields across a large area, such as the mouth of a harbor, it is known to string a plurality of such point detection magneto meters together so as to create a detection wall for detecting incoming ships or submarines. However, the complexity and cost of such strings of magnetometers prohibits their use in commercial and industrial applications.
It is an object of the present invention to provide an improved apparatus, method and system which overcomes the above noted problems of the prior art for detecting the presence of any part of a train within a block.
It is a further object of the present invention to provide an apparatus, method and system in which a fiber optic magnetometer is used for detecting the presence of any part of a train within a block.
It is a still further object of the present invention to provide an apparatus, method and system in which a single fiber optic cable pair, comprising a sense arm and a reference arm, is placed along the length of a block, and form a sensor for detecting the presence of any part of the train within a block.
It is a still yet a further object of the present invention to provide an apparatus, method and system in which a single fiber optic cable pair comprising a sense arm and a reference arm form a distributed sensor for detecting magnetic field fluctuation over any portion of the distributed sensor.